1. Field of the Invention
The present invention relates to a photodiode provided for use under avalanche operating conditions at ambient temperature, and including a PN junction surrounded by a guard ring and made from an Hg.sub.1-x Cd.sub.x Te crystal.
Such a photodiode may be used, for example, for receiving optical signals propagating along optical fibers. In fact, the avalanche operating conditions provide a step-up of the photocurrent, by avalanche, which confers a good sensitivity on the photodiode. Furthermore, operating at ambient temperature makes possible a use which would be too heavy with a photodiode which has to be cooled.
2. Description of the Prior Art
Photodiodes of this type are already known from the French Pat. No. 81 04712, whose maximum sensitivity corresponds to a wave length between 0.8 .mu.m and 2 .mu.m.
These known photodiodes are therefore not adapted to use with optical fibers made from fluorated glass which, in the spectral band situated between 2 and 4 .mu.m, will theoretically have extremely low losses of the order of 10.sup.-3 dB/km, and which should allow links of several hundreds of kilometers in length to be constructed without receivers.
In order to obtain an Hg.sub.1-x Cd.sub.x Te photodiode which is sensitive in a certain spectral band, it is known to adjust the molar fraction x of the material which is used. In fact, the molar fraction x controls the width of the forbidden band or "gap" of the material and so its spectral response.
Thus, from the patent U.S. Pat. No. 4,137,544 a Hg.sub.1-x Cd.sub.x Te photodiode is known whose molar fraction x is chosen equal to 0.2, so that the spectral response of the photodiode corresponds to the 8-14 .mu.m range. For this value of the molar fraction x, the width of the forbidden band is very small and does not allow operation at ambient temperature. Such a photodiode therefore requires a cooling device. As is known, the same goes for all the Hg.sub.1-x Cd.sub.x Te photodiodes whose molar fraction is less than substantially 0.35. Moreover, this diode is not adapted for operating under avalanche conditions, particularly because it has no guard ring about the PN junction.
An Hg.sub.1-x Cd.sub.x Te photodiode, whose molar fraction x is chosen equal to 0.4 so that the spectral response is centered on 2.7 .mu.m, is described by S. H. SHIN et al in "High Performance Epitaxial Hg Cd Te Photodiode for 2.7 .mu.m Applications", IEEE Electron Devices Letters, vol. EDL-2, no. 7, July 1981, pages 177-179, New York USA. This photodiode operates at ambient temperature, but it is not adapted for operating under avalanche conditions, particularly because it has no guard ring.
The modification of the two preceding photodiodes so as to be able to cause them to operate under avalanche conditions is a priori unthinkable, considering the fact that the biasing in the avalanche region of a junction of small forbidden band width, leads to the creation of a dark current due to the tunnel effect which is too high to allow satisfactory operation of the photodiode. This problem is in particular mentioned by HIROAKI ANDO et al in the article "INGaAS/InP Separated Absorption and Multiplication Regions Avalanche Photodiode using Liquid and Vapor Phase Epitaxies" pages 250-254 IEEE Journal of Quantum Electronics vol. QE-17, no. 2, February 1981.
In order to overcome the above problems, the prior art teaches abandoning conventional photodiodes for photodiodes having separated absorption and multiplication regions. In these photodiodes, the photons are absorbed in a material having a small forbidden band width. The electron-hole pairs thus created diffuse as far as the junction situated in a material of wider forbidden band width. The junction is biased in the avalanche region so as to multiply the photocurrent created in the region of small forbidden band width. Such a photodiode is however complex in design and manufacture.